PROJECT SUMMARY/ABSTRACT Based on current diagnoses rates, one in two Black men-who-have-sex-with-men (BMSM) will be diagnosed with HIV during their lifetime. Interventions to optimize retention in care and adherence to antiretroviral therapy (ART) among BMSM are urgently needed to reduce HIV racial disparities. Dyadic intervention approaches are promising, given that nearly half of BMSM living with HIV are in primary relationships, and are consistent with calls for research with BMSM to focus beyond the individual level. Our preliminary research with BMSM in couples finds that the primary relationship is often the main resource in meeting needs in HIV care engagement. BMSM couples engage in joint problem-solving and dyadic coordination of care that entail synchronizing and co-organizing schedules, activities, and behaviors around treatment and care. We also found that dyadic coordination and joint problem-solving in care engagement may be more common among BMSM couples with greater couple?s resilience, defined as the couple?s capacity to overcome shared adversity that buffers against the strains of social and structural factors on health and wellbeing for BMSM. Based on our preliminary work and extant research, we developed a theoretical Framework of Dyadic HIV Care Engagement to guide this research. Smartphones are ubiquitous among BMSM of various ages and socioeconomic backgrounds, and mobile technology for health (mHealth) is feasible, acceptable, and effective as HIV interventions among BMSM. We have developed initial designs for a BMSM-couple-focused intervention application (app), LetSync, to improve retention in care and ART adherence. BMSM couples showed a great interest in this approach. However, critical issues remain, such as the need for a deeper understanding of how dyadic processes interrelate to bolster dyadic capacity, how they may be targeted in an mHealth intervention, and the need for piloting intervention components and features. As such, the aims to this R01 are as follows. Aim 1 will involve investigating interrelated dyadic processes in HIV care engagement (couple?s resilience, joint problem-solving, dyadic coordination) through in-depth interviews with BMSM in 12 seroconcordant-positive and 12 serodiscordant couples (24 couples, N=48) guided by our conceptual framework. Aim 2 will develop a couple?s mHealth intervention informed by Aim 1 and our prior mHealth designs, through iterative app development with a community advisory council of BMSM couples and a mini-pilot of the app prototype, intervention protocols, and procedures. Aim 3 will entail a randomized, waitlist-controlled pilot trial with 80 couples (N=160) to assess its acceptability, feasibility, and preliminary impact on retention in care and ART adherence as measured by antiretroviral concentrations in hair. Findings from the proposed research are critical for informing a subsequent full-scale RCT to test the efficacy of LetSync in improving HIV care and treatment outcomes among BMSM couples toward reducing HIV racial disparities in the U.S.